Although known for over 90 years, only in the past two decades has the chemistry of diboron(4) compounds been extensively explored. Many interesting structural features and reaction patterns have emerged, and more importantly, these compounds now feature prominently in both metal-catalyzed and metal-free methodologies for the formation of B–C bonds and other processes.

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

The empty pz-orbital of a three-coordinate organoboron compound leads to its electron-deficient properties, which make it an excellent π-acceptor in conjugated organic chromophores. The empty p-orbital in such Lewis acids can be attacked by nucleophiles, so bulky groups are often employed to provide air-stable materials. However, many of these can still bind fluoride and cyanide anions leading to applications as anion-selective sensors. One electron reduction generates radical anions. The π-acceptor strength can be easily tuned by varying the organic substituents. Many of these compounds show strong two-photon absorption (TPA) and two-photon excited fluorescence (TPEF) behaviour, which can be applied for e.g. biological imaging. Furthermore, these chromophores can be used as emitters and electron transporters in OLEDs, and examples have recently been found to exhibit efficient thermally activated delayed fluorescence (TADF). The three-coordinate organoboron unit can also be incorporated into polycyclic aromatic hydrocarbons. Such boron-doped compounds exhibit very interesting properties, distinct from their all-carbon analogues. Significant developments have been made in all of these areas in recent years and new applications are rapidly emerging for this class of boron compounds.

https://pubs.rsc.org/en/content/articlepdf/2017/SC/C6SC04245G

Recent developments in and perspectives on three-coordinate boron materials: a bright future.

Shortly after their discovery at the end of the 80s, stable singlet carbenes have been recognized as excellent ligands for transition metal based catalysts, and as organo-catalysts in their own right. At the end of the 2000s, it has been shown that they can coordinate main group elements in their zero oxidation state, and even activate small molecules. This review covers examples in the literature dealing with the most recent application of stable singlet carbenes, namely their use to stabilize radicals and radical ions that are otherwise unstable.

/pdf/carbene-stabilized-main-group-radicals-and-radical-ions-3vtqqe3pi1.pdf

Carbene-stabilized main group radicals and radical ions

Oxidative addition and reductive elimination are key steps in a wide variety of catalytic reactions mediated by transition-metal complexes. Historically, this reactivity has been considered to be the exclusive domain of d-block elements. However, this paradigm has changed in recent years with the demonstration of transition-metal-like reactivity by main-group compounds. This Review highlights the substantial progress achieved in the past decade for the activation of robust single bonds by main-group compounds and the more recently realized activation of multiple bonds by these elements. We also discuss the significant discovery of reversible activation of single bonds and distinct examples of reductive elimination at main-group element centers. The review consists of three major parts, starting with oxidative addition of single bonds, proceeding to cleavage of multiple bonds, and culminated by the discussion of reversible bond activation and reductive elimination. Within each subsection, the discussion is...

Oxidative Addition and Reductive Elimination at Main-Group Element Centers

Perfluorarylborane Lewis acids catalyse the addition of silicon-hydrogen bonds across C=C, C=N and C=O double bonds. This 'metal-free' hydrosilylation has been proposed to occur via borane activation of the silane Si-H bond, rather than through classical Lewis acid/base adducts with the substrate. However, the key borane/silane adduct had not been observed experimentally. Here it is shown that the strongly Lewis acidic, antiaromatic 1,2,3-tris(pentafluorophenyl)-4,5,6,7-tetrafluoro-1-boraindene forms an observable, isolable adduct with triethylsilane. The equilibrium for adduct formation was studied quantitatively through variable-temperature NMR spectroscopic investigations. The interaction of the silane with the borane occurs through the Si-H bond, as evidenced by trends in the Si-H coupling constant and the infrared stretching frequency of the Si-H bond, as well as by X-ray crystallography and theoretical calculations. The adduct's reactivity with nucleophiles demonstrates conclusively the role of this species in metal-free 'frustrated-Lewis-pair' hydrosilylation reactions.

/pdf/direct-observation-of-a-borane-silane-complex-involved-in-3xrq3mhu2p.pdf

Direct observation of a borane–silane complex involved in frustrated Lewis-pair-mediated hydrosilylations

The element boron is known to have a variety of ways to relieve its inherent electron deficiency. The acceptance of an electron pair (Lewis acidity) has applications in catalysis and activation of element–element bonds (frustrated Lewis pairs). The combination of boron with p-donating substituents (e.g. BF3) and its incorporation into organic p-conjugated systems allows the empty pz orbital of boron to participate in p bonding and p conjugation, respectively, and the latter enables the use of boron in optoelectronic materials with unique properties. The absence of p-donating substituents at the boron center may result in multiple-center bonding to form nonclassical frameworks (e.g. B2H6 or clusters). In addition, organoboranes and -diboranes(4) are prone to accept a single electron by chemical reduction. Likewise, hydrogen atom abstraction from N-heterocyclic carbene(NHC)-stabilized boranes (NHC-BH3) can lead to neutral, persistent boryl radicals of the type NHC-BH2C, [5] which have been studied by means of cyclic voltammetry, EPR, and UV/Vis spectroscopy as well as trapping reactions. However, examples of isolated boron radicals are rare owing to the reactive nature of the species, and only little is known about their structural properties. Steric protection of the boron center combined with spin delocalization over the organic substituents, both achieved by substitution with mesityl groups (Mes= 2,4,6-trimethylphenyl), has occasionally enabled isolation and structural characterization of radical anions such as [Li([12]crown-4)2][BMes3] (1) or [K([18]crown-6)(thf)2][Mes2BB(Ph)Mes] (2). [7] Our group has recently studied a persistent radical anion as an intermediate in the stepwise reduction of 1-ferrocenyl2,3,4,5-tetraphenylborole (3). Boroles are a class of antiaromatic compounds with interesting chemical and photophysical properties that are well-known for their ability to accept two electrons with formation of an aromatic borole dianion. Encouraged by these recent results on the radical anion [3]C , which indicated the presence of a highly unusual C4B p system bearing five electrons, [8] we set out to isolate and characterize a stable borol radical anion. As we report here, this was possible by choice of steric protection and an appropriate reducing agent. The synthesis of MesBC4Ph4 (1-mesityl-2,3,4,5-tetraphenylborole, 4) by means of the commonly employed tin–boron exchange reaction was unsuccessful because of the low reactivity of dihalo(mesityl)boranes (MesBX2; X=Cl, Br). However, 4 was obtained in 41% yield by functionalization of the boron center in 1-chloro-2,3,4,5-tetraphenylborole (5) through nucleophilic displacement of the chlorine ligand with LiMes (Scheme 1). A more efficient alternative was found to be the salt-elimination reaction of MesBCl2 with 1,4-

An isolable radical anion based on the borole framework.

We have exploited the reactivity of antiaromatic boroles, gaining access to aryl-substituted monocyclic 1,2-azaborinines. The observed ring-expansion reaction of inherently electron-deficient boroles with organometallic and organic azides is demonstrated for representative examples. This substance class is expected to provide a new avenue into 1,2-azaborinine chemistry, especially in the area of functional organoboron materials. Our results are based on NMR and UV/Vis spectroscopy as well as single-crystal X-ray crystallography and provide a virtually quantitative approach that also offers numerous points of variation.

https://chemistry-europe.onlinelibrary.wiley.com/doi/pdf/10.1002/chem.201404051

Antiaromaticity to aromaticity: from boroles to 1,2-azaborinines by ring expansion with azides

As neutral isoelectronic analogues of the elusive cyclopentadienyl cation, boroles have been of interest for their prospective applications as strong Lewis acids, chromophores, and electron acceptors. Recently our group discovered a π-nucleophilic boryl anion based on the borole system. In an effort to extend borole chemistry, we now report the molecular structure of 1-chloro-2,3,4,5-tetraphenylborole (1) and its corresponding borole dianion resulting from the two-electron reduction of 1 with KC 8 . The thermally induced dimerization of 1 yields an unprecedented boracyclohexadiene/borolene spiro-bicyclic compound and the resulting dimer was fully characterized including a single-crystal X-ray analysis.

Chemical Reduction and Dimerization of 1‐Chloro‐2,3,4,5‐tetraphenylborole

Unwinding Antiaromaticity in 1-Bromo-2,3,4,5-tetraphenylborole

Free borole chemistry has witnessed enormous progress in the past years that led to new insights into their bonding, antiaromaticity, and reactivity. While the early research in the field was mainly stimulated by fundamental questions regarding their 4π-electron antiaromaticity, boroles are increasingly in the focus of current research activities due to their unusual electronic and optical properties, which arise from the overlap of the vacant pz-orbital on boron with the diene system in the five-membered ring. This chapter gives a systematic and comprehensive account of free borole chemistry with a particular emphasis on substituent effects on their molecular properties.

Johannes Wahler

Papers

An isolable radical anion based on the borole framework.

Antiaromaticity to aromaticity: from boroles to 1,2-azaborinines by ring expansion with azides

Chemical Reduction and Dimerization of 1‐Chloro‐2,3,4,5‐tetraphenylborole

Unwinding Antiaromaticity in 1-Bromo-2,3,4,5-tetraphenylborole

Free Boroles: The Effect of Antiaromaticity on Their Physical Properties and Chemical Reactivity